Most DDR3 memory modules available today can handle 2GB while 4GB modules are hard to find in stock. If you are about to increase the memory capacity further you would have to get another 2GB module, which in the case of the Intel Core i7 platform would result in up to 12GB RAM. Memory manufacturer Samsung now offers three times the capacity with a single module. Samsung's new DDR3 memory targets Intel servers and can store up to 32GB data.

You could have every single temp, cache folder in RAMdisk... (can you have the entire OS in RAMdisk?)

But apart from Photoshop and CAD, is there any application that would benefit from this much RAM?

Jep, 4Gb is the absolute maxium for 32-bit OS'es.
Having the complete OS in RAMdisk might be possible, but with every reboot you'll have to copy a complete OS from disk to memory and after each reboot you'll have to copy the content of the RAM back to the disk and you loose everything when power is lost or the system crashes, but other than that it'll be fast !

Professional sound and video editing software might benefit from the extra ram too ... if 64bit editions are available tough